The present invention relates to a system for adding small amounts of material to a flowing stream or process using the loss-in-weight principal. The term `loss-in-weight` is used to designate a system such as those described in U.S. Pat. Nos. 3,099,368 and 3,452,774 for adding small quantities of materials to process streams or processes. Both of the patents describe systems in which the material is placed in a hopper or other container and continuously weighed as the material is metered from the hopper into the process stream. The disclosed systems use analog circuits and control a flow means such as a valve or the like to meter the material to the process stream. While the systems are satisfactory, the use of a valve which must be accurately positioned to meter the material can cause problems in refinery and chemical plants where corrosive materials are used. For example, in catalytic reforming the catalyst must be periodically regenerated by a process which requires the uniform addition of small amounts of chlorine. Due to the corrosive nature of chlorine, conventional metering systems do not operate successfully. This is particularly true of systems such as those shown in the prior art patents that utilize flow control valves for metering the addition of the material. The valves require accurate positioning which is impossible in a corrosive atmosphere.